It is necessary to cool alternators of automotive vehicles, and similar machines while they are operating, so as to limit their operating temperature and so ensure optimum performance. Such cooling is usually obtained by a cooling fan comprising a rotor which gives forced ventilation through the machine. It is necessary that the inevitable increase in selling price resulting from the need to provide a cooling fan rotor in the machine should be as small as possible.
One of the known methods by which such cooling fan rotors are made is described especially in European published patent application No. EP 0 270 393A. In such a method, the first operation is to stamp out an approximately circular sheet metal blank having on its periphery the same number of teeth as there are to be fins or blades in the cooling fan. In a second operation these teeth are bent at right angles to the blank, so forming the cooling blades.
Although such a method allows cooling fan rotors to be made very cheaply, it nonetheless has certain drawbacks, notably that only a limited number of blades can be formed. In fact, the number of cooling blades which it is possible to obtain by this method depends on the length of the circumference of the sheet metal blank; and even if a sufficiently large number of blades can be formed in a machine of large diameter, this is not true where the diameter of the machine is small, as is the case for example in alternators for automotive vehicles. For these reasons, the working surface of the cooling fins is correspondingly limited.
It is possible to remedy these disadvantages to some extent, by giving the cooling fins or blades particular shapes such as to increase their performance, but these methods do inevitably lead to additional manufacturing operations which result in increased cost; and indeed, these operations are in any case often incompatible with mass production techniques.